The sex steroid hormones estradiol and progesterone regulate physiology and behavior, including female sexual behavior, in rodents and other species. The expression of the lordosis posture in response to genital stimulation is dependent upon a neural action of estradiol and progesterone of during the guinea pig estrous cycle. Estradiol binding to estrogen receptors causes the accumulation occupied estrogen receptors in cell nuclei of cells within certain brain regions including the hypothalamus, preoptic area, midbrain central gray, and amygdala. Similarly, progesterone binding to progestin receptors causes the accumulation of tightly-bound progestin receptors in cell nuclei of target cells. The accumulation of cell nuclear receptors has been postulated to be a critical event in the cellular mechanism of action of the hormones. The focus of this research is on how steroid hormone- sensitive neurons operate and interact with other neurons in the regulation of sexual behavior. The long-range objectives of this research are to use a multi-pronged approach to study the neural actions of sex steroid hormones resulting in changes in behavior. The emphasis will be on the mechanisms of hormone action related to progesterone-facilitated sexual behavior. The role in progesterone-facilitated sexual behavior in guinea pigs of progestin receptor-containing neurons in specific neuroanatomical areas will be examined. An attempt will be made to identify neurons that are activated directly or indirectly by estradiol or progesterone in parts of the brain which are involved in progesterone-facilitated sexual behavior, and an attempt will be made to determine the importance of particular pathways in progesterone-facilitated sexual behavior. Neurons that are activated by copulatory stimulation will be identified, and their properties will be characterized. For example, it will be determined if these neurons contain steroid receptors as would be expected of neurons whose function is to integrate environmental and hormonal information. The regulation of progestin receptors by noradrenergic neurons will be investigated further. The distribution of progestin receptor-immunoreactive cells having noradrenergic input will be mapped, using both light and electron microscopic techniques. Finally, the hypothesis that vagino-cervical stimulation activates certain neurons via noradrenergic receptors will be tested.